sw_params for a direct-ed(dmix) hw pcm

[Takashi Iwai]

On Thu, 26 Mar 2020 21:04:15 +0100,
sylvain.bertrand at gmail.com wrote:
> 
> On Thu, Mar 26, 2020 at 03:36:23PM +0100, Jaroslav Kysela wrote:
> > I agree. Also, the snd_pcm_direct_sw_params() does nothing, because the
> > sw_params are already cached in the pcm structure (see comment). It means
> > that the dmix (direct) plugins operates with those cached values. Just set
> > sw_params like for any other PCM handle. The dmix uses those values (if
> > possible).
> 
> This is the "if possible" which would impacts the way how code should do setup
> right, but:
> 
> Let's take the case of a classic plugin "pipeline":
> pcm:plug->...->direct::dmix->hw
> 
> >From the top plugin (usually plug) to the direct::plugin, the "sw_params" pcm
> op is usually pcm_generic.c:snd_pcm_generic_sw_params which does recurse down.
> This recursion down will stop once pcm_direct.c:snd_pcm_direct_sw_params is
> reached, then will recurse up, without error.
> 
> But pcm.c:snd_pcm_sw_params will copy anyway the provided sw_params into each
> recursed back pcm if the "sw_params" pcm op return no error code, which is the
> case.
> 
> Then looking at pcm.c:snd_pcm_sw_params_current, I get those "wrong" sw_params,
> then I get no way to know something went wrong.
> 
> Why "wrong", because they may significantly differ from the bottom hw plugin
> sw_params which some fields are used to configure the kernel driver.
> 
> for instance, a fast_op status call will recurse down to
> pcm_dmix.c:snd_pcm_dmix_status, which will call the hw plugin fast op status
> function which will use _its_ tstamp_type field for the ioctl call, but will
> "override" the trigger_tstamp field computed with the "wrong" sw_params
> tstamp_type!
> 
> It happens that the monotonic_raw and monotonic clocks can have audio
> significant difference. Additionally, the other sw_params field might cause
> similar issues.

The tstamp type handling in dmix is certainly buggy, yes.
It should have been restricted with the slave PCM unless it's
compatible.


Takashi